Question
I am noticing more and more that it is growing on the top of my soil in pots and now climbing up the trunk of my tree fern. The colour is green and in a clump formation which seems to spread. Each individual spore seems to be 3mm across and has small volcanic like pits in it.It is quite flat in its appearance. Do you have any ideas please.

Answer
This is pushing the edges of my area of knowledge, but here are some thoughts...

Green flat growth on the surface of soil and up stems seems more likely to be algae than fungus. It would dry out to be black and crusty. It likes hard-packed, moist soil with lots of fertilizer. Ugly but should not harm plants much. It should be made out of filaments, however.

Mold or fungus on soil is usually white or grey and stringy, though the fruiting bodies may be round. There is a fungus called Artillery fungus that makes small volcano-shaped fruiting bodies, but they are not green. They shoot black round dots of spores way up in the air which stick to leaves and houses and are extremely hard to remove. Soil molds like acidic and dry conditions, but then fruit when the dried out soil is wetted.  

In either case, the best thing to do is to remove the growth and either turn it into the soil or compost it elsewhere. If your pots seem to be chronically wet, you need to reduce watering and improve drainage (perhaps with holes beneath covered by gravel, covered by soil.

If your pots seem to be chronically dry or smell musty when you water them, you need to increase the moisture content a bit, and might benefit from a bit of lime.

If you have large pots with a layer of mulch on top, make certain to keep the mulch under 2 inches deep.

In general, whenever you are adding soil, mulch, or any organic material to any planting situation (pot, ground, whatever) you need to make sure the whole material is damp before or while applying it, and you need to make certain the area will drain reasonably well.

Bacteria also grow in soil, and they help keep mold in check. Some come in a greenish color, but I've never heard of one getting so thick you would see it on the surface. I suppose moss or lichen could be flat and green, but I think your most likely answer is Algae.